1. Field of the Invention
The present invention relates to an electron emission device, and more particularly to an electron emission device having an electron emission element which emits electrons by application of a voltage and an anode electrode which absorbs the emitted electrons.
2. Related Background Art
In a prior art electron emission device, one of various electron emission elements to be described later is used and electrons emitted thereby are attracted by an anode electrode to form an electron flow.
The electron emission element used is one which uses an avalanche breakdown of a PN junction (PN-A type), one which injects electrons into a P layer by applying a forward bias to a PN junction (PN-B type), one which has a thin insulation layer sandwiched by metal layers (MIM type), an electric field emission type element or a surface conduction type element
FIG. 1A shows a forward bias applied to a PN junction to inject electrons into a P layer, and FIG. 1B shows a current-voltage characteristic thereof.
In FIG. 1A, when a forward bias V is applied to the PN junction, a forward current I follows as shown in FIG. 1B, and the electrons injected from an N layer 102 to a P layer 101 are emitted from a surface 103 of the P layer into vacuum. On the surface of the P layer, a work function reducing material 104 such as cesium C.sub.s is applied in order to increase the amount of electron emission.
FIG. 2 shows an MIM type electron emission element, and FIG. 3 shows a surface conduction type electron emission element.
The MIM type electron emission element has a laminated structure of a metal electrode 201, an insulation layer 202 and a thin metal electrode 203. By applying a voltage across the electrodes 201 and 203, electrons are emitted from the thin electrode 203.
The surface conduction type electron emission element has electrodes 302 and 303 formed on an insulative substrate 301 and a high resistance thin film 304 formed therebetween. By applying a voltage between the electrodes 302 and 303, electrons are emitted from the surface of the high resistance thin film 304.
FIG. 4 shows a prior art electron emission device which uses an electron emission element as shown in FIG. 1A. As shown in FIG. 4, a voltage V.sub.1 is applied to the electron emission element 401 by a power supply 403 and the emitted electrons are attracted to an anode electrode 402 to which a voltage V.sub.2 is applied from a power supply 404 to form an electron flow 405.
In such prior art electron emission devices, the amount of electron emission changes with a change of external environment or a change of efficiency of electron emission of the element and hence a stable electron current is not attained. As shown in FIG. 1B, when the application voltage V is higher than V.sub.f, the current I changes significantly and it is difficult to stabilize the amount of electron emission.
When the MIM type or surface conduction type electron emission element is used, the change of the current is significant when the applied voltage is higher than a predetermined level and it is difficult to stabilize the electron current, as is the case for the PN type element. Further, since the amount of electron emission changes with the change of external environment and the change of efficiency of electron emission of the element, a stable electron flow is not attained.
The instability of the electron flow is also observed in the element which uses the avalanche breakdown of the PN junction or the electric field type element.
The Cs applied on the electron emission surface in order to lower the work function is an unstable element and causes a change of the amount of electron emission.